Which military activity by the viet cong caused american troops the most difficulty?

We really didnt have a advantage in the Vietnam war. We were more technological advanced, but they knew the land and used gurella warfare. The viet cong traveled at night and hide at day. they stocked supplys in civilian homes so when we came to seize them the civilians would grow angry at us. And in the battles we were usually greatly outnumbered due to the fact that we traveled mostly by helicopter which had a limited seating capacity therefor it would be 30 minutes to multiple hours before reinforcements arrived so the engagement could last for a couple hours to multiple days.